There is a difference between unlevel/sunken inserts... and sagging/cupped inserts.
Always level inserts with heat gun and re-glue or you'll have problems again in the future. The insert needs to be "secure" in its forever home or it will eventually work it's way out.
Cupped inserts either need to be "melted" back in shape as BubbaK suggested; and/or use some kind of filler to level the shallow areas.